Coaxial cavity tracking means



Feb, 6, 1962 L. BExsER ETAL 3,020,499

COAXIAL CAVITY TRACKING MEANS Filed May 20, 1960 2 Sheets-Sheet 1 fhgh fhigh fo fo Af O @m flow flow o l flow f fm@ o l F/ G 5 /N VENT URS LEO BEISER SEYMOUR SCHNEIDER THE/R ATTRNEY Feb. 6, 1962 L. BEISER ErAL 3,020,499

COAXIAL CAVITY TRACKING MEANS Filed May 20. 1960 2 Sheets-Sheet 2 York Filed May 20, 1960, Ser. No. 30,490 7 Claims. (Cl. S33- 82) The present invention relates to coaxial resonators, and more particular to new and novel tracking means for incrementally tuning the coaxial cavity to comply with a given error function.

Many microwave apparatus, in particular, field intensity meters and receivers, include a plurality of separate tunable circuits, and it often becomes important to be able to tune these various circuits simultaneously While maintaining them in some predetermined relationship. For example, in a tuned high frequency type of receiver, various tuned circuits must be maintained with equal resonant frequency. ln staggered-tuned circuits, such tuned circuits must be ideally maintained with a constant difference between the resonant frequencies. Also, in the su-v perheterodyne type of microwave receiver, a local oscil lator tuned circuit and a preselector tuned circuit must be tunable over the entire range of reception for the receiver, while simultaneously maintaining a relatively constant frequency difference ybetween the resonant frequenf cies of the respectively tuned circuits. In the microwave field these tuned circuits generally are of the cavity resonator type wherein the resonant frequency of a cavity is varied by changing the edective electrical length of the transmission line defined by the cavity dimensions. Thus a typical microwave receiver might include an external cavity klystron local oscillator, a tracking preselector employing two resonant coaxial cavities coupled by means of apertures to yield both the ,desired bandwidth and the desired rejection of spurious signals, and other track circuit components such as dial mechanisms, a potentiometer to vary the klystron repeller voltage,etc.

In many such applications as above described the tuned circuit consists of a coaxial resonator wherein the tuning is accomplished by the axial translation of the moveable center conductor with respect to the outer conductor of the cavity. ln such cases the variation in resonant frequency is a non-linear function of the displacement of the control member, i.e., the moveable center conductor, and moreover this frequency vs. displacement rclationship may differ for the various tuned circuits to be simultaneously controlled. Accordingly, this gives rise to a problem known as tracking, by which is meant the maintenance of the respective tuned circuits in their proper resonant frequency relationship over the entire range of variation in tuning. Thus, i-n the case of the cavity resonators employed in the klystron local oscillator and the preselector of the typical microwave receiver above-mentioned, where the respective resonators are tuned to dif-v ferent frequencies requiring different movement of their controlling members lfor the same frequency change, the relationship of frequency vs. center conductor displacement for each cavity is 'a complexlyvariable one over the range of frequencies involved. Therefore relativelyelaborate means must be provided for tuning the various resonant cavities and other tracked elements in the microwave apparatus in order to ensure that the desired relationship over the tuning range exists between the respective frequencies, tuning dial indicators, repeller potentiometer, etc. One such means for accomplishing this tracking embraces the use of an initial or coarse cam for adjusting the displacement of the center conductor in the resonant coaxial cavity to approximately tune the device. To obtain optimum performance it then belit States Patent O cornes essential that the resonant frequency of the cavity be adjusted by 4a fine correction cam which has many individual adjustment points and which superimposes an individual corrective effect upon the tuning of the cavitiesy by the coarse cam in acting upon the displacement of the center conductor wtih respect to the outer conductor.A

The inclusion of this last named cam, which accomplishes the ne tuning of the coaxial cavity to comply with a given error function in order to maintain the proper tracking relationship, is a technique in general use today in microwave'equipment wherein this tracking pro-blem is present. Mechanical tracking of this type suffers from many disadvantages among them the requirement that the drive assembly be critically assembled to close dimensions and tolerances with resultant costly machining and alignment procedures. Also the complicated mechanical linkages involved in transferring the compensation adjustment from the maindrive to the incremental adjustment of the tuning' plunger in the coaxial cavity inherently leads to residual backlash and dead spots. Further, such mechanical mechanisms are relatively expensive to fabricate because of the large number of components required. In many applications such as airborne equipment the great weight and size of the mechanical tracking adjustment means is a serious factor.

The present invention provides a novel electrical tracking compensator for a coaxial cavity, such as those used in preselectors and other staggered-tuned circuits, to incrementally adjust the resonant frequency of the cavity to comply with a given error function; i.e., to accomplish the ne frequency tuning of a tracking tuned circuit. The coarse or major resonant tuning is first eected by a variation of the active length of the center conductor insideV the coaxial cavity by means of a couventional mechanical displacement linkage. The track` ing compensation -is then achieved by the use of a plurality of individually adjustable members staggered in space relationship along the outer conductor wall of the coaxial cavity in conjunction with a modification of the center conductor or tuning plunger such that the electrical field configuration in the vicinity of the end point of the tuning plunger is influenced by the depth of penetration f Y of the particular `adjustable member in registerk with the end point of the cen-ter conductor to thereby provide an incremental lvariation of the cavity field geometry and thus of its` resonant frequency. The invention provides an yelectrical tracking compensator which has the advantages of simplicity; economy; high resolution; and very significantly, a relative independence of adjustment and effect of each discreet tine tuning member. The present invention is particularly useful in aligning and correcting the tracking of lsuch tuned circuits which' are mass produced in quantity, wherein unavoidable variations in machining tolerances and electrical characteristics maybe found, and which may thus be readily compensated for by the presentinvention. j

It is therefore an objective of the invention to provide a new and novel means for tracking a coaxial cavity resonator.'

It is another object of the invention to provide a new' and novel means for incrementally tuning a resonant coaxial cavity -to comply with a given error function.

',It is yet another objective of this invention to provide' a tracking compensator which is particularly suitable for. use in multi-stage preselectors and other staggered micro- Y wave tuned circuits where it is desirable to maintain a economical to fabricate, simple in construction, and of much smaller size and weight than conventional mechanical tracking compensators.

And a still further object of the present invention is to provide a new and novel tracking compensator for tunable coaxial cavities having a plurality of individually adjustable tuning corrections which are relatively independent of each other.

Further other objects and advantages of the present invention will become apparent upon consideration of the following drawings wherein FIGURES 1, 2, and 3, are representative plots of certain curves which will be useful in explaining the theory of operation of the invention;

FIGURE 4 is a front view, partially sectional, of a coaxial cavity resonator incorporating the present invention;

FIGURE 5 is a cross-sectional view as seen substantially along the plane of the line 5--5 in FIGURE 4;

FIGURE 6 is a front view, partially sectional, of an alternative form of cavity resonator according to the present invention; and

FIGURE 7 is a cross-sectional view as seen substantially along the plane of the line 7-7 in FIGURE 6.

The problem of the tracking of a coaxial cavity resonator so that it may be incrementally tuned to comply with a given error function may be understood if reference is made to the following illustrative curves of FIGURES 1, 2, and 3. FIGURE l is a representative plot of the resonant frequency fo vs. the active length l of the center conductor of a typical coaxial cavity resonator. As the tuning plunger or center conductor is axially translated with reference to the outer conductor of the cavity the resonant frequency thereof varies in the manner shown from a relatively high resonant frequency for a small active length of the plunger inside the cavity to a relatively low resonant frequency as the center conductor is extended to its fullest penetration inside the coaxial cavity. It is to be noted that although the curve is non-linear, it is nevertheless smoothly continuous and represents an inverse frequency function, as may be seen.

FIGURE 2 is a representative curve of the error function Af vs. frequency which arises in other tracked components of the microwave system due to reference miscalibration; i.e., the incremental error which arises in the alignment of the tracked components of the system,

such as the multiple tuned circuits having different tuning characteristics, the variation of the potentiometer adjustment controlling the klystron repeller voltage, the dial mechanism, etc. Thus FIGURE 2 is representative of the tracking error which might result in a microwave system having a number of tracked components after the assemblyY of components and their gross alignment, by means of coupled mechanical translating linkages (such as the cam and follower mechanisms above-described), is made. The error function Af vs. frequency f for the microwave system may not follow any determinable relationship but may vary randomly around the desired operating point Af=0g i.e., perfect tracking.

FIGURE 3 is a super-position of the error function Af quency fo vs. active length l of the center conductor inside the coaxial cavity resonator. A combination of the two yields a plot of the desired variation in the center conductor travel vs. frequency for the coaxial cavity resonator which would result in a perfect tracking of the system. Thus FIGURE 3 is suggestive of the prior art or mechanical method of providing tracking for a tuned cavity-incremental variation of the active length of the center conductor to achieve frequency correction by means of a ne correction cam having many individual adjustment points or other similar mechanical means for achieving individual corrective effect upon the gross displacement by the main cam of the tuning plunger or center conductor with respect to the outer conductor of the coaxial cavity resonator.

' 16a16n and the relative spacing therebetween.

The present invention, however, provides an electrical tracking means, whereby the equivalent of corrective motion of the center conductor is accomplished by an incremental variation of the cavity geometry, defined by the center conductor-to-wall spacing, insofar as the electrical field configuration in the vicinity of the end point of the tuning plunger is concerned. Incremental tuning correction and hence tracking compensation are achieved by the resulting change in the capacity of the resonant cavity as the center conductor-to-wall spacing is varied. The error correcting information is applied directly to the electrical field within the cavity such that a correction is accomplished at the frequency of interest.

Referring again to the drawings, and particularly to FIGURES 4 and 5 thereof, there is shown therein a coaxial cavity resonator, designated generally as 10, having an outer conductor 11 with an end wall 12 and a center conductor or tuning plunger 14. Apertures 13 are provided for inserting and extracting energy from the resonator in a conventional manner as is well understood by those cognizant of the art. Arranged along a line substantially parallel to the longitudinal axis of the outer conductor 11 and projecting therethrough, are a series of metallic members, designated as 16a-1611, in spaced relationship. Each of these members 16a-16n, which are shown, for example, in the form of a threaded set screw having a chamfered tip portion 17 and a slotted head 18, are capable of adjustment such that the depth of penetration of each threaded member into the cavity may be individually varied with relative facility by rotation of a member 16 through means of its slotted head 18. The tuning plunger or center conductor 14 is terminated at the end of the portion Within the cavity by a disc-shaped member 20 of substantially larger diameter than the main body diameter of the center conductor tuning plunger 14. The clearance between the tip portion 17 of particular metallic member 161' and the peripheral edge of the discshaped member 20 is determinative of the amount of capacitive coupling between these two elements with the result that such clearance is a measure of the degree of electrical eld distortion in the region of the end point of the tuning conductor 14.

An axial displacement of this center conductor 14 within the cavity by gross tuning means (not shown) accomplishes the major tuning of the resonant frequency of the resonator 10. As such displacement takes place, it may readily be seen that disc 20 terminating the center conductor 14 passes along under the plurality of metallic members 16a-16n. The end-loaded capacity of the tuning plunger 14 to the outer conductor wall 11 of the cavity is thus a direct function of the depth of penetration of the particular metallic member 161 immediately opposite the peripheral edge of the disc 20. Any variation of the clearance between this peripheral edge and the tip 17i of the particular metallic member 161' which is juxtaposed immediately thereabove would vary the electrical field configuration in the immediate vicinity of the terminating end of the center conductor 14, and hence slightly affect (in comparison to the major tuning control) the resonant frequency fo of the cavity as will be readily understood by those conversant with the art. It may therefore be seen that a ne tuning of the resonator frequency can be achieved by selective adjustment of the depth of penetration of the various individual metallic members 16a- 1611.

Metallic members immediately adjacent the particular member 161' juxtaposed above disc termination 20, such members being designated for purposes of generality as 16i1 and 16H-1, may have some effect on the actuation of the incremental tuning at the point of interest depending upon the relative thickness of the disc 20 with respect to the cross-sectional diameter of the metallic members This slight degree of carry-over adjustment from point to point may be necessary and desirable in order to avoid a scalloping effect between metallic members due to discontinuous carry-over from one to the next as the tuning plunger is axially translated. A disc-shaped configuration is preferred for the member 20, although other forms are utilizable in this invention, since such a shape has the advantage of insensibility of the tuning correction, as determined by the depth of penetration of the particular member 161', to rotation of the center conductor 14, since the clearance between the peripheral disc and the tip portion 171' remains constant by reason of the axial symmetry of the terminating disc 20. Other terminating conligurations may be utilizable in the practice of this invention, such as conical, parabolic, etc., forms, in an eort to obtain loptimum control over the end-loaded capacity of the tuning plunger consistent with independence of adjustment.

Furthermore the metallic members 16a-1611 may be spatially disposed in other than a generally straight line path parallel to the longitudinal axis of the resonator cavity l0; for example, the axial progression may be in the form of a helical curve about the outer conductor.

Such an arrangement is provided in the alternative form of the invention illustrated in FIGURES 6 and 7 wherein screws 16a through 16u are arranged on all four sides of cavity 11 along a helical path. Elements in FIGURES 6 and 7 have the same function as in the previous figures and are given the same referencernumbers. One advantage of the helical arrangement is .the virtually unlimited number of adjusting screws which may be provided as desired. Obviously the helical arrangement may be replaced with a staggered or random `arrangement with similar advantages.

Thus the invention provides a means of tracking compensation over a plurality of points throughout the frequency range in interest with only a small degree of interdependence of adjustment from point to point. By selective adjustment of each individual metallic member juxtaposed opposite the terminating disc 20 at the particular resonant frequency fo at which it is desired to have resonant cavity track the microwave system in which it is incorporated, it is possible to virtually reduce the error function A f to zero (corresponding to perfect tracking) at a number of frequency points equal to the quantity n, corresponding to the number of metallic members 16. This result lis accomplished by a slight controllable distortion of the electrical field geometry in the vicinity of each one of the individual members la-ln, in the manner above-described, thereby to superimpose a fine tuning frequency correction on the major tuning of the coaxial cavity 10, as determined by the active length l of the center conductor 14 within the cavity walls.

It is thought that the foregoing description, if taken in conjunction With the Iannexed drawings, will be sucient to enable anyone practiced in the art to understand and construct a coaxial cavity tracking compensator in accordance with the principles of this invention. Accordingly, it is desired that the invention be taken to embrace any variation and embellishment of the particular embodiment shown which is within the spirit thereof and that the scope of this invention be limited solely by the following appended claims.

What is claimed is:

1. A tracking compensator for the incremental adjustment of the resonant frequency of a tunable coaxial cavity comprising, a coaxial cavity having an outer conductor and a center conductor relatively moveable thereto along their common longitudinal axis, a metallic discshaped member affixed near the end point of said center conductor Within said cavity, a plurality of individually adjustable metallic members spaced in substantially axial progression along said outer conductor and projecting therethrough to a controllable depth of penetration into said cavity, such that at least one of said individually adjustable members in the vicinity of the endpoint of said center conductor electrically interacts, substantially independently of the remainder of said individually-adjustlable metallic members, with said'disc-shaped member at a plurality of operating positions to vary the end loaded capacity of said center conductor to said outer conductor wall to provide thereby a relatively fine adjustment in the frequency tuning of said cavity .at said plurality of operating positions.

2. The combination set forth in claim l, said discshaped member being characterized by a diameter substantially greater than the cross-sectional diameter of said center conductor.

3. The combination set forth in claim 2, said discshaped member being further characterized by having a thickness on the same order of magnitude as the thickness of asid individually adjustable metallic members, said second-named thickness dimension being measured in a direction parallel to the longitudinal axis of said cavity.

4. The combination set forth in claim 1, said axial pro.

gression of said individlally adjustable metallic members being substantially in the form of a helix about said outer conductor of said cavity.

5. The combination set forth in claim 1, said axial progression of said individually-adjustable metallic members being substantially in a straight line path parallel to said common longitudinal axis.

6. The combination set forth in claim 1, said individually-adjustable metallic members being substantially uniformly distributed longitudinally of said common longitudinally axis and at least partially distributed circumferentially of said common longitudinal axis.

7. A tracking compensator for incremental adjustment of the resonant yfrequency of a tunable coaxial cavity comprising, a coaxial cavity having an outer conductor -and a center conductor relatively moveable thereto along their common longitudinal axis, a metallic disc-shaped member axed near the endpoint of said center conductor within said cavity, a plurality of individually adjustable conductive members spaced in axial progression along said outer conductor and projecting therethrough to a controllable depth of penetration into said cavity, such that at least one of said individually adjustable members in the vicinity of the endpoint of said center conductor cooperates, substantially independently of the remainder of said individually-adjustable members, with said discshaped member at a plurality of operating positions to vary the effective electrical eld configuration in said vicinity without physically contacting said disc-shaped member to provide thereby a relatively line adjustment -in the frequency tuning of said cavity at said plurality of operating positions.

References Cited in the tile of this patent UNITED STATES PATENTS 2,572,232 Wolfe Oct. 23, 1951 2,600,278 Smullin June 10, 1952 2,639,406 Crawfordv May 19, 1953 2,837,722 Goublin-Korsten June 3, 1958 2,851,666 Kach Sept. 9, 1958 

